Cathode and battery including same

ABSTRACT

A cathode and a battery including the cathode are provided. The cathode includes a cathode mixture layer with a cathode active material and a binder. The binder can include, for example, a synthetic rubber latex and a thickener, polyvinylidene fluoride denaturalized by maleic acid and/or the like.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention claims priority to Japanese Patent Document No.P2002-215168 filed on Jul. 24, 2002, the disclosure of which is hereinincorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to cathodes and batteries including same.More specifically, the present invention relates to cathodes having acathode mixture layer including a cathode active material and a binder,and batteries including the cathode.

Recently, in connection with the development of electronic engineering,many compact portable electronic devices, such as a combination camera(e.g., video tape recorder), a mobile phone, and a laptop computer arecommonly known and used, and the size and weight of such devices arebeing reduced. Consequently, as a portable power source used to powersave, a compact and lightweight battery with a high energy density,particularly, a secondary battery has been developed.

For example, a secondary battery having an anode active material capableof inserting and extracting lithium metals, lithium compounds, orlithium ions, has the high voltage and excellent reversibility. Inparticular, a lithium ion secondary battery, using a composite oxide oflithium and a transition metal as a cathode active material, and using acarbonaceous material as an anode active material, is lightweight andhas a large discharge capacity, compared to conventional lead secondarybatteries and nickel-cadmium secondary batteries. Thus, the lithium ionsecondary battery is widely used for electronic devices, such as mobilephones, laptops and the like.

Currently, a primary example of typically used cathode active materialsfor the lithium ion secondary battery is LiCoO₂. There exist, however, anumber of problems related to use of same, such as, in terms of loadcharacteristics, charge and discharge cycle characteristics, and safetyor the like. For example, in order to improve the load characteristics,it is necessary to smooth an electrode, further to make the electrodeinto a thin film. To obtain such electrode, it is necessary to downsizegrain diameters of the materials making the electrode, and to improveconductivity. However, when downsizing the grain diameters, the specificsurface area becomes large. Thus, unless more binder is added, theelectrode becomes fragile, and sufficient peel strength cannot beobtained.

However, polyvinylidene fluoride (PVDF), which has been conventionallyand primarily used as a binder, is a non-electrically conductivepolymer. Therefore, there exists a problem such that increasing theamount of PVDF causes not only lowering of a ratio of an active materialin the electrode and lowering of a charge and discharge capacity, butalso hindrance of electron transfer, increase of internal resistance ofthe electrode, and significant deterioration of charge and dischargecycle life of the battery and capability of high load charge anddischarge of the battery. Further, there exists another problem suchthat the electrode becomes hard and fragile, and electrode peeling andcracking occur.

A need therefore exists to provide improved batteries, including partsthereof, such as cathodes.

SUMMARY OF THE INVENTION

The present invention provides a cathode, wherein a high-strength thinfilm electrode can be realized, its load characteristics are improved,its charge and discharge capacity and capacity maintenance ratio arehigh, and its charge and discharge cycle life are enhanced, and abattery including same.

A cathode according to an embodiment of the present invention includes acathode mixture layer containing a cathode active material and a binder.The cathode mixture layer contains a synthetic rubber latex adhesive anda thickener as the binder. In the cathode mixture layer, the content ofthe synthetic rubber latex adhesive ranges from about 2 wt % to about 4wt %, and the content of the thickener ranges from about 0.5 wt % toabout 2.5 wt %.

A cathode according to an embodiment of the present invention comprisesa cathode mixture layer containing a cathode active material and abinder. The cathode mixture layer contains polyvinylidene fluoridedenaturalized by maleic acid as the binder, wherein the content of thepolyvinylidene fluoride in the cathode mixture layer ranges from about0.5 wt % to about 4 wt %.

A battery according to an embodiment the present invention includes acathode, an anode, and an electrolyte. The cathode includes a cathodemixture layer containing a cathode active material, and synthetic rubberlatex adhesive and a thickener as the binder. In the cathode mixturelayer, the content of the synthetic rubber latex adhesive ranges fromabout 2 wt % to about 4 wt %, the content of the thickener ranges fromabout 0.5 wt % to about 2.5 wt %, and a charge final voltage is about4.0 V and under.

A battery according to an embodiment of the present invention includes acathode, an anode and an electrolyte. The cathode includes a cathodeactive material and polyvinylidene fluoride denatured by maleic acid asthe binder. In the cathode mixture layer, the content of polyvinylidenefluoride denatured by maleic acid ranges from about 0.5 wt % to about 4wt %, and a charge final voltage is about 4.0 V and under.

In the cathode, according to an embodiment of the present invention,since the synthetic rubber latex adhesive and the thickener are includedas a binder, high flexibility and smoothness can be obtained. Thus,electrode peeling and cracking are prevented. Further, the syntheticrubber latex adhesive and the thickener, or polyvinylidene fluoridedenaturalized by maleic acid is contained as a binder according to anembodiment of the present invention. Thus, the content of the binder canbe decreased, and the ratio and capacity of the active material can beincreased. Further, electron transfer can be facilitated to decrease theresistance.

In a battery according to an embodiment of the present invention, acathode according to an embodiment of the present invention is employed.Thus, the charge and discharge capacity is increased and the internalresistance is decreased. Further, the charge and discharge capacity andthe charge and discharge cycle life are improved, and the loadcharacteristics are improved.

Additional features and advantages of the present invention aredescribed in, and will apparent from, the following Detailed Descriptionof the Invention and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an exploded perspective view of a secondary battery accordingto an embodiment of the invention.

FIG. 2 is a cross sectional view taken along line I-I of a batteryelement illustrated in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The present invention generally relates to batteries and parts thereof.More specifically, the present invention relates to cathodes that have acathode mixture layer that includes a cathode active material and abinder, and batteries including same.

FIG. 1 shows an exploded view of a secondary battery according to anembodiment of the invention. FIG. 2 shows a cross sectional view takenalong line I-I of a battery element 20 illustrated in FIG. 1. Thepresent invention will be described below, without limitation, ingreater detail with reference made to the figures where appropriate.

In a secondary battery according to an embodiment of the presentinvention, the battery element 20, wherein a cathode 21 and an anode 22are layered and wound with an electrolyte layer 23 and a separator 24 inbetween, is crimped and enclosed in film exterior members 30 a and 30 b.The exterior members 30 a and 30 b are made of, for example, an aluminumlaminated film wherein a polyolefin film, an aluminum foil, and apolyolefin film are applied together in this order. Outer edges of theexterior members 30 a and 30 b are contacted with each other. An end ofa cathode lead wire 11 and an end of an anode lead wire 12 are projectedfrom the exterior members 30 a and 30 b. Adhesive film 31 is insertedbetween the exterior members 30 a and 30 b and the cathode lead wire 11and the anode lead wire 12, for example. The adhesive film 31 is used tosecure the insulation, as well as to improve the adhesion propertiesbetween the cathode lead wire 11 and the anode lead wire 12 and theexterior members 30 a and 30 b.

In the battery element 20 according to an embodiment of the presentinvention, for example, the separator 24, the electrolyte layer 23, thecathode 21, the electrolyte layer 23, the separator 24, the electrolytelayer 23, the anode 22, and the electrolyte layer 23 are sequentiallylayered and wound. At the outermost circumferential part, for example, aprotective tape 25 is adhered. The cathode lead wire 11 is connected tothe cathode 21 of the battery element 20, and the anode lead wire 12 isconnected to the anode 22. The cathode lead wire 11 and the anode leadwire 12 can be respectively made of a metal or an alloy havingconduction. For example, it is preferable that the cathode lead wire 11is made of aluminum and the anode lead wire 12 is made of nickel. Itshould be appreciated that other suitable materials or combinationsthereof can be used.

The cathode 21 is, for example, composed of a cathode current collectorlayer 21 a and cathode mixture layer 21 b, having a structure whereincathode mixture layer(s) 21 b is provided on both sides or single sideof the cathode current collector layer 21 a. The cathode currentcollector layer 21 a is made of metal foil, such as aluminum foil,nickel foil, stainless foil, the like or combinations thereof. Thecathode mixture layer 21 b contains, for example, a cathode activematerial such as lithium phosphorous oxide or the like and a binder adescribed below, for example, and may additionally contain a conductiveagent. Further, the cathode mixture layer 21 b is not provided on oneend part of the cathode current collector layer 21 a, so that the endpart is exposed. The cathode lead wire 11 is attached to the exposed endpart.

The lithium phosphorous oxide has, for example, an olivine structure. Itcontains, in an embodiment, at least one first element, such asmanganese (Mn), chromium (Cr), cobalt (Co), copper (Cu), nickel (Ni),vanadium (V), molybdenum (Mo), titanium (Ti), zinc (Zn), aluminum (Al),gallium (Ga), magnesium (Mg), boron (B), niobium (Nb), iron (Fe), andthe like; lithium; phosphorous; oxygen and the like and combinationsthereof. The lithium phosphorous oxide is expressed by, for example, achemical formula of Li_(x)MPO₄ (0<x≦1.2), wherein M represents the firstelement. Examples include LiFe_(0.2)Cu_(0.8)PO₄, LiFe_(0.9)Ti_(0.1)PO₄,LiFe_(0.8)Zn_(0.2)PO₄, LiFe_(0.8)Mg_(0.2)PO₄, the like and combinationsthereof. In particular, lithium-iron-phosphorous composite oxides, whichis readily available and inexpensive, are preferable.

The lithium phosphorous oxide provides excellent characteristics in thecase where a charge final voltage is controlled to be about 4.0 V orless. Binders described later start to be decomposed when the chargefinal voltage exceeds about 4.0 V. However, in the case where thelithium phosphorous oxide is used as a cathode active material and thecharge final voltage is controlled to be about 4.0 V or less, highbattery characteristics can be obtained, which is preferable. As acathode active material, other materials capable of controlling thecharge final voltage to be about 4.0 V or less can be used. It is alsopossible to combine such materials with the lithium phosphorous oxide.

An average grain diameter of the cathode active material is, forexample, preferably in the range from about 0.5 μm to about 3 μm. Inthis regard, it is believed that the cathode 21 can be smoothed and madeinto a thin film by downsizing the average grain diameter. Further, byusing the binders described later, even when the average grain diameteris downsized, the flexible and smooth cathode 21 can be obtained withoutincreasing a ratio of the binder.

It is preferable to use, for example, a synthetic rubber latex adhesiveand a thickener as a binder. It is believed that by using them, theflexible and smooth cathode 21 can be obtained, and electrode peelingand cracking can be prevented. Further, it becomes possible to reducethe amount of the binder compared to in conventional cases, and toincrease the ratio of amount of the cathode active material. Moreover,electron transfer can be facilitated, and the internal resistance of thebattery can be reduced.

Examples of the synthetic rubber latex adhesive are styrene butadienerubber latex, nitrile-butadiene rubber latex, methyl methacrylatebutadiene rubber latex, chloroprene rubber latex and the like, or thelike. Any one of them or a mixture of two or more of them may be used.Examples of the thickener are synthetic polymers, such as polyacrylicacid, polyethylene oxide, polyvinyl alcohol, polyacrylamide; celluloseether resins such as methyl cellulose, ethyl cellulose, triethylcellulose, carboxymethyl cellulose, carboxyethyl cellulose, aminoethylcellulose; or natrium salt type or ammonium salt type of these celluloseether resins. Any one of them or mixture of two or more of them may beused. It is preferable to use ammonium salt type polyacrylic acid withsalt tolerance among the above materials, considering it is relativelyinexpensive and easy to use.

It is preferable that, the content of the synthetic rubber latexadhesive in the cathode mixture layer 21 b, ranges from about 2 wt % toabout 4 wt % with respect to all mass of the cathode mixture layer 21 b;and the content of the thickener in the cathode mixture layer 21 branges from about 0.5 wt % to about 2.5 wt % with respect to all mass ofthe cathode mixture layer 21 b. When the content of the synthetic rubberlatex adhesive exceeds the above range, viscosity is remarkably raisedin forming the cathode mixture layer 21 b, so that application to thecathode current collector layer 21 a becomes difficult; and when thecontent of the synthetic rubber latex adhesive is below theabove-described range, the cathode mixture layer 21 b becomes fragile,so that sufficient strength cannot be obtained. When the content of thethickener exceeds the above range, gelation is significant in formingthe cathode mixture layer 21 b, so that application to the cathodecurrent collector layer 21 a becomes impossible; and when the content ofthe thickener becomes less, the cathode mixture layer 21 b becomefragile, so that sufficient strength cannot be obtained.

It is also preferable to use polyvinylidene fluoride denaturalized bymaleic acid (hereinafter referred to as maleic acid-denaturalizedpolyvinylidene fluoride) as another binder according to an embodiment ofthe present invention. It is believed that, by using this, an amount ofadhesive can also be reduced compared to in conventional cases, sincepeel strength is improved. A maleic acid-denaturalized amount preferablyranges from about 0.1 wt % to about 0.4 wt %. When a denaturalizedamount exceeds the above range, gelation is significant in forming thecathode mixture layer 21 b, so that application to the cathode currentcollector layer 21 a becomes impossible; and when a denaturalized amountbecomes less, the cathode mixture layer 21 b become fragile, so thatsufficient strength cannot be obtained.

Further, it is also preferable to use a material obtained bysubstituting part of maleic acid-denaturalized polyvinylidene fluoridewith hexafluoro propylene (HFPr) (hereinafter referred to asHFPr-substituted maleic acid-denaturalized polyvinylidene fluoride) as abinder. It is believed that the application aspect of the cathodemixture is improved, a higher discharge capacity can be obtained, andthe cycle characteristics can be improved. Substitution ratio ofhexafluoro propylene is preferably about 5 wt % and under, since whenthe ratio exceeds 5 wt %, though peel strength can be obtained,absorption of electrolyte solution is intense, peeling due to charge anddischarge is easy to occur, and cycle life is easy to lower.

It is preferable that a content of maleic acid-denaturalizedpolyvinylidene fluoride or HFPr-substituted maleic acid-denaturalizedpolyvinylidene fluoride in the cathode mixture, in an embodiment, rangesfrom about 0.5 wt % to about 4 wt %. When a content exceeds the aboverange, gelation in forming the cathode mixture layer 21 b issignificant, so that application to the cathode current collector layer21 a becomes impossible; and when a content becomes less, the cathodemixture layer 21 b become fragile, and sufficient strength cannot beobtained.

Examples of conductive agents are carbon materials such as carbon black,e.g. Ketjen black, graphite the like or combinations thereof. Suchcarbon material is preferably contained within the cathode activematerial, and the content in an embodiment preferably ranges from about5 wt % to about 12 wt % with respect to the total amount of the cathodeactive material and the carbon material. When the content is less thanabout 5 wt %, the conductivity decreases, and significant deteriorationof the load characteristics and deterioration of charge and dischargecapacity occur; and when the content exceeds about 12 wt %, the ratiobecomes excess in relation to the cathode active material, bulk densityof the cathode mixture layer 21 b is large, and further increasingcontent of the binder becomes necessary, both of which are notpreferable.

The anode 22 has a structure, in an embodiment, wherein an anode mixturelayer(s) 22 b is provided on both sides or single side of an anodecurrent collector layer 22 a respectively in a manner similar to in thecathode 21. The anode current collector layer 22 a is made of metalfoil, such as copper foil, nickel foil, stainless foil and/or the like.The anode mixture layer 22 b contains, for example, any one kind or twoor more kinds of anode materials capable of inserting and extractinglithium as the electrode active material, and may additionally contain abinder such as polyvinylidene fluoride as necessary. The anode mixturelayer 22 b are not provided on one end part of the anode currentcollector layer 22 a, so that the end part is exposed. The anode leadwire 12 is attached to this exposed end part.

Examples of anode materials capable of inserting and extracting lithiumare carbon materials, metal oxides, high molecular weight materials andthe like. As the carbon materials, for example, there are pyrolyticcarbons, cokes, graphite, glassy carbons, organic high molecular weightcompound fired bodies, carbon fibers, spherical carbons, or activatedcarbons and the like. The cokes include pitch cokes, needle cokes,petroleum cokes and the like. The organic high molecular weight compoundfired bodies denote ones obtained by firing and carbonizing highmolecular weight materials such as a phenol resin, a furan resin or thelike at appropriate temperature. The carbon fibers include a mesophasecarbon fiber or the like. The spherical carbons include mesophase carbonmicro beads or the like. Examples of the metal oxides are iron oxide,ruthenium oxide, or molybdenum oxide or the like. Examples of the highmolecular weight materials are polyacetylene, polypyrrole, or the like.

As an anode material capable of inserting and extracting lithium,substances, alloys, or compounds of metal elements or semimetalelements, which are capable of forming alloys with lithium. The alloysinclude, in an embodiment, two or more metal elements, and, in addition,alloy-based materials that include one or more metal elements and one ormore semimetal elements. In the structure of each of the materials,solid solution, eutectic (eutectic mixture), or intermetallic compoundexists, or two or more of them coexist.

Examples of such metal elements or semimetal elements include tin (Sn),lead (Pb), aluminum (Al), indium (In), silicon (Si), zinc (Zn), antimony(Sb), bismuth (Bi), gallium (Ga), germanium (Ge), arsenic (As), silver(Ag), hafnium (Hf), zirconium (Zr), and yttrium (Y) and the like. Analloy or compound of these elements is expressed by, for example, achemical formula of M_(s)Mb_(t)Li_(u), or a chemical formula ofMa_(p)Mc_(q)Md_(r). In an embodiment, Ma represents at least one ofmetal elements and semimetal elements capable of forming an alloy withlithium, Mb represents at least one of metal elements and semimetalelements other then lithium and Ma, Mc represents at least one ofnon-metal elements, and Md represents at least one of metal elements andsemimetal elements other than Ma. The values of s, t, u, p, q, and rsatisfy s>0, t≧0, p>0, q>0, and r≧0, respectively.

In an embodiment, a substance, an alloy, or a compound of a group 4Bmetal element(s) and/or semimetal element(s) is preferable. In anembodiment, silicon and tin, and their alloys and compounds arepreferred in crystalline or amorphous state.

Examples of such alloys and compounds are LiAl, AlSb, CuMgSb, SiB₄,SiB₆, Mg₂Si, Mg₂Sn, Ni₂Si, TiSi₂, MoSi₂, CoSi₂, NiSi₂, CaSi₂, CrSi₂,CU₅Si, FeSi₂, MnSi₂, NbSi₂, TaSi₂, VSi₂, WSi₂, ZnSi₂, SiC, Si₃N₂O,Si_(v) (0<v≦2), SnO_(w) (0<v≦2), SnSiO₃, LiSiO, LiSnO and the like.

Any one kind or mixture of two or more of the above can be used for ananode material capable of inserting and extracting lithium according toan embodiment of the present invention.

The electrolyte layer 23 includes, in an embodiment, an electrolytesolution wherein the lithium salt as the electrolyte salt is dissolvedin a nonaqueous solvent; and a gel electrolyte containing a highmolecular weight material.

Examples of the lithium salts include LiPF₆, LiBF₄, LiClO₄, LiCF₃SO₃,LiN (CF₃SO₂)₂, LiN (C₂F₅SO₂)₂ and the like. One of them, or mixture oftwo or more of them can be used.

The nonaqueous solvents include, for example, ethylene carbonate,propylene carbonate, butylene carbonate, γ-butyrolactone,γ-valerolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane,tetrahydrofuran, 2-methyl tetrahydrofuran, 1,3-dioxyline, methylacetate,methyl propionate, dimethyl carbonate, diethyl carbonate, ethyl methylcarbonate, 2,4-difluoro anisole, 2,6-difluoro anisole, 4-bromoveratroleand the like. One or mixture of two or more of the nonaqueous solventscan be used.

The high molecular weight materials include polyvinylidene fluoride,polyacrylonitrile, polyethylene oxide, or polymethacrylonitrile and thelike. One of them, or mixture of two or more of them can be usedcorresponding to type of usage.

As the separator 24, for example, an insulative thin film having largeion permeability and a given mechanical strength is used. Specifically,a porous thin film made of a polyolefin material, such as polypropylene,polyethylene and the like, or a porous thin film made of an inorganicmaterial such as a ceramic non woven cloth is used. It is possible tolayer two or more kinds of such porous thin films. A thickness of theseparator 24 preferably ranges from about 1 μm to about 30 μm,considering the mechanical strength and the battery capacity.

In an embodiment, a secondary battery can be produced as describedbelow.

First, for example, a cathode active material, a binder, and aconductive agent as necessary are mixed. The mixture is dispersed in asolvent such as N-methyl-2-pyrrolidone to thereby obtain the cathodemixture slurry. As a binder, as mentioned above, the synthetic rubberlatex adhesive and the thickener, or maleic acid-denaturalizedpolyvinylidene fluoride, or HFPr-substituted maleic acid-denaturalizedpolyvinylidene fluoride can be used in an embodiment. After producingthe cathode mixture slurry, for example, the cathode mixture slurry isapplied on both sides or single side of the cathode current collectorlayer 21 a, dried, and compression molded to form the cathode mixturelayer 21 b. In such a manner, the cathode 21 is produced. Then, thecathode mixture slurry is not applied on one end part of the cathodecurrent collector layer 21 a, and the end part is exposed.

Subsequently, for example, an anode active material, a binder, and aconductive agent as necessary are mixed to prepare an anode mixture. Themixture is dispersed in a solvent such as N-methyl-2-pyrolidone tothereby obtain the anode mixture slurry. After producing the anodemixture slurry, for example, this anode mixture slurry is applied onboth sides or single side of the anode current collector layer 22 a,dried, and compression molded to form the anode mixture layer 22 b. Insuch a manner, the anode 22 is produced. Then, the anode mixture slurryis not applied on one end part of the anode current collector layer 22a, and the end part is exposed.

After producing the cathode 21 and the anode 22, for example, thecathode lead wire 11 is attached to the exposed part of the cathodecurrent collector layer 21 a, and the anode lead wire 12 is attached tothe exposed part of the anode current collector layer 22 a respectivelyby resistance welding, ultrasonic welding or the like.

Subsequently, for example, the electrolyte layer 23 made of a gelelectrolyte is formed on the cathode mixture layer 21 b and the anodemixture layer 22 b. The electrolyte layer 23 are, for example, formed bymixing an electrolyte solution, a high molecular weight material, anddimethyl carbonate which is a solvent of this high molecular weightmaterial; applying this mixture on the cathode mixture layer 21 b or theanode mixture layer 22 b; drying it, and volatilizing the solvent.

After forming the electrolyte layer 23, for example, as shown in FIG. 2,the separator 24, the cathode 21 formed with the electrolyte layer 23,the separator 24, and the anode 22 formed with the electrolyte layer 23are sequentially layered and wound, and the protective tape 25 is, forexample, adhered at the outermost circumferential part. In this manner,the battery element 20 is formed.

After producing the battery element 20, the exterior members 30 a and 30b made of e.g. aluminum laminated films are prepared, between which thebattery element 20 is sandwiched. In the reduced-pressure atmosphere,the exterior members 30 a and 30 b are crimped to the battery element20, which is enclosed by contacting outer edges of the exterior members30 a and 30 b by heat anastomoses or the like. Then, for example, theadhesive film 31 is inserted between the cathode lead wire 11 and theanode lead wire 12 and the exterior members 30 a and 30 b, and thecathode lead wire 11 and the anode lead wire 12 are projected from theexterior members 30 a and 30 b. In this manner, assembly of thesecondary battery is completed. A shape of the secondary battery is notlimited to the shape shown in FIGS. 1 and 2, and other shapes arecontemplated within the scope of the present invention.

After assembling the secondary battery, for example, the secondarybattery is heated to higher temperature than normal temperatures whilebeing uniaxial pressurized. Namely, the battery element 20 is heatedwhile being pressurized through the exterior members 30 a and 30 b. Inthis way, the electrolyte solution contained in the electrolyte layer 23is penetrated in the cathode mixture layer 21 b and the anode mixturelayer 22 b, so that the adhesion between the electrolyte layer 23 andthe cathode 21 and the anode 22 is raised. Additionally, the adhesionbetween each electrode active material is raised, and the contactresistance of the electrode active material is lowered. Through theabove processes, the secondary battery is completed.

In the secondary battery, when charge is performed, for example, lithiumions extract from the cathode 21, and are inserted into the anode 22 viathe electrolyte layer 23 and the separator 24. When discharge isperformed, for example, lithium ions extract from the anode 22, and areinserted into the cathode 21 via the electrolyte layer 23 and theseparator 24. In this case, an amount of the binder contained in thecathode mixture layer 21 b is suppressed to be a small amount, and thecontent of the electrode active material is increased. Therefore,internal resistance in the battery is reduced, the charge and dischargecapacity and the charge and discharge cycle life are improved, andparticularly the load characteristics are improved.

The charge final voltage is about 4.0 V or less in an embodiment. Whenit exceeds about 4.0 V, the above-mentioned binder contained in thecathode mixture layer 21 b is decomposed, thereby causing lowering ofthe cycle characteristics and load discharge capacity. However, when alithium phosphorous oxide is used as a cathode active material,excellent battery characteristics can be obtained with the charge finalvoltage of about 4.0 V or less.

As discussed above, in an embodiment, effective amounts of the syntheticrubber latex adhesive and the thickener are contained in the cathodemixture layer 21 b as a binder. Therefore, the flexible and smoothcathode 21 can be obtained, and electrode peeling and cracking can beprevented. Additionally, effective amounts of the synthetic rubber latexadhesive and the thickener, or maleic acid-denaturalized polyvinylidenefluoride are contained, so that a content of the binder lowers, theratio of the cathode active material in the cathode mixture layer 21 band the capacity of the cathode 21 can be increased, and electrontransfer in the cathode 21 can be facilitated. Therefore, the charge anddischarge capacity, and the charge and discharge cycle life can beimproved, and the load characteristics can be improved.

In particular, in the case where denaturalized amount of maleicacid-denaturalized polyvinylidene fluoride ranges from about 0.1 wt % toabout 0.4 wt %, and part of maleic acid-denaturalized polyvinylidenefluoride is substituted with hexafluoropropylene of about 5 wt % orless, a higher discharge capacity can be obtained, and the cyclecharacteristics and the high load characteristics can be furtherimproved.

Examples of the present invention, without limitation, will be describedbelow in greater detail with reference to FIGS. 1 and 2.

Examples 1-1 and 1-2

First, as a cathode active material, lithium iron phosphorous oxide(LiFePO₄) as the lithium phosphorous oxide was prepared under thefollowing conditions. Lithium phosphate and phosphorous iron (II) oxideoctahydrate were mixed at an element ratio of lithium:iron=1:1. Ketjenblack powders were added to the mixture, so that its ratio was 10% ofthe whole fired material obtained after firing, thereby obtaining amixed sample. This mixed sample was put in an alumina container, andmilling was made by a planetary ball mill at a weight ratio ofsample:alumina ball that equaled 50%, at a number of revolutions of 250rpm, and for operation time of 10 hours. After that, the mixed samplewas put in a ceramic pot, and was fired in an electric furnace in thenitrogen atmosphere at 600° C. for 5 hours, thereby obtaining a firedmaterial of LiFePO₄ containing a carbon material.

The cathode mixture was prepared by using the LiFePO₄ as the cathodeactive material, sufficiently kneading the fired material of LiFePO₄containing the carbon material, and ammonium salt polyacrylic acid (PAA)in a planetary mixer, and adding styrene butadiene rubber latex (SBR) tothe mixture. Then, a mass ratio of the fired material of LiFePO₄containing the carbon material:ammonium salt polyacrylic acid:styrenebutadiene rubber latex was (99−x):1:x. The value of x was varied asshown in Examples 1-1 and 1-2 in Table 1. The cathode mixture wasdispersed in N-methyl-2-pyrrolidone, thereby obtaining the cathodemixture slurry. Then, the cathode mixture slurry was uniformly appliedon both sides of the cathode current collector layer 21 a made of astrip-shaped aluminum foil, and dried. After that, compression moldingwas performed by a roller pressing machine, thereby forming thestrip-shaped cathode mixture layer 21 b, which were cut in a given sizeto produce the sheet cathode 21.

TABLE 1 Mass ratio of total SBR PAA PVDF amount mass mass mass of Peelstrength Binder ratio x ratio ratio y binder test Example 1-1 SBR + 2 1— 3 Sufficient PAA Example 1-2 SBR + 4 1 — 5 Strong PAA ComparativeSBR + 1 1 — 2 Insufficient Example 1-1 PAA Comparative SBR + 5 1 — 6Meas- Example 1-2 PAA urement impossible Comparative PVDF — — 3 3Insufficient Example 1-3 Comparative PVDF — — 6 6 Sufficient Example 1-4

Next, mesophase carbon micro beads as the anode active material andpolyvinylidene fluoride as a binder were mixed at a mass ratio of 90:10,to thereby prepare an anode mixture. Subsequently,N-methyl-2-pyrrolidone as the solvent was added to this anode mixture,which was then stirred and mixed to obtain an anode mixture slurry.Subsequently, the anode mixture slurry was uniformly applied on bothsides of the anode current collector layer 22 a made of a strip-shapedcopper foil and dried. After that, compression molding was performed bya roller pressing machine to form the strip-shaped anode mixture layer22 b, which were cut in a given size to produce the sheet anode 22.

Further, LiPF₆ was dissolved in a solvent wherein ethylene carbonate andpropylene carbonate were mixed to produce an electrolyte solution. Afterthat, the electrolyte solution, a high molecular weight material, anddimethyl carbonate as the solvent of this high molecular weight materialwere mixed and stirred to obtain a gel electrolyte.

After producing the cathode 21, the anode 22, and the electrolyte, thecathode lead wire 11 was attached to the cathode current collector layer21 a, the electrolyte was applied to the cathode mixture layer 21 b,dimethyl carbonate was evaporated, thereby forming the electrolyte layer23. Additionally, the anode lead wire 12 was attached to the anodecurrent collector layer 22 a, the electrolyte was applied to the anodemixture layer 22 b, dimethyl carbonate was evaporated, thereby formingthe electrolyte layer 23. Then, a pair of separator 24 made of amicro-porous polypropylene film with a thickness of 9 μm was prepared.The separator 24, the cathode 21, the separator 24, and the anode 22were sequentially layered in this order and wound, and the protectivetape 25 was adhered, thereby obtaining the battery element 20. Thebattery elements 20 were produced for Examples 1-1 and 1-2 respectively.

Peel strength tests were conducted for the battery elements 20 ofExamples 1-1 and 1-2 produced as above, based on JIS B 7721. Obtainedresults are shown in Table 1. The peel strength tests in Table 1 wereevaluated as “Strong” when binding force is 7 gf/mm and above;“Sufficient” when binding force is 2 gf/mm and above, and less than 7gf/mm; and “Insufficient” when binding force is less than 2 gf/mm.

As Comparative Examples 1-1 and 1-2 to be compared with Examples 1-1 and1-2, the battery elements 20 were produced in a manner similar toExamples 1-1 and 1-2, except that the value of x in (99−x):1:x, which isa mass ratio of the fired material of LiFePO₄ containing a carbonmaterial to ammonium salt polyacrylic acid to styrene butadiene rubberlatex, was varied as shown in Table 1. Further, as Comparative Examples1-3 and 1-4 to be compared with Examples 1-1 and 1-2, the batteryelements 20 were produced in a manner similar to Examples 1-1 and 1-2,except that a binder made of polyvinylidene fluoride (PVDF) was usedinstead of the binder made of ammonium salt polyacrylic acid and styrenebutadiene rubber latex, and the cathode mixture was made at a mass ratioof a fired material of LiFePO₄ containing a carbonmaterial:polyvinylidene fluoride=(100−y):y. The value of y was varied asshown in Comparative Examples 1-3 and 1-4 in Table 1. ComparativeExample 1-4 is a typical example of a binder contained in a conventionalcathode mixture. The peel strength tests were also conducted forComparative Examples 1-1 to 1-4 in a manner similar to Examples 1-1 and1-2. Obtained results are shown in Table 1.

As evidenced by Table 1, sufficient binding force was obtained inExample 1-1, and strong binding force was obtained in Example 1-2. InComparative Example 1-1, however, sufficient binding force could not beobtained. It is believed, for example, that this result was due to theamount of styrene butadiene rubber latex as the binder that was toosmall. In Comparative Example of binder 1-2, viscosity of the cathodemixture was significantly raised, and application to the cathode currentcollector layer 21 a could not be performed well. It is believed, forexample, that this resulted because the amount of styrene butadienerubber latex as the binder was too great. In Comparative Example 1-3,containing polyvinylidene fluoride of 3 wt %, no binding force wasobserved, and significant peeling was shown at the electrode. InComparative Example 1-4, containing polyvinylidene fluoride of 6 wt %,though sufficient binding force was obtained, the content of the binderwas too great.

In this regard, it was found that by using ammonium salt polyacrylicacid and styrene butadiene rubber latex as a binder, and setting thecontent of styrene butadiene rubber latex to the range from about 2 wt %to about 4 wt %, an amount of binder became smaller, and the cathode andthe secondary battery with sufficient binding force, for example, highstrength could be obtained.

Examples 2-1 and 2-2

The rolled battery elements 20 were produced in a manner similar toExample 1-1, except that the value of z in (98−z):z:2, which is a massratio of a fired material of LiFePO₄ containing a carbon material toammonium salt polyacrylic acid to styrene butadiene rubber latex, wasvaried as shown in Table 2. The peel strength tests were conducted forthe battery elements 20 of Examples 2-1 and 2-2 made as above in similarway to Example 1-1. Obtained results are shown in Table 2. The peelstrength tests in Table 2 were evaluated in a manner similar to Table 1.

TABLE 2 Mass ratio SBR PAA of total mass mass amount Peel strengthBinder ratio ratio z of binder test Example 2-1 SBR + PAA 2 0.5 2.5Sufficient Example 2-2 SBR + PAA 2 2.5 4.5 Sufficient Comparative SBR +PAA 2 0.3 2.3 Insufficient Example 2-1 Comparative SBR + PAA 2 3 5Measurement Example 2-2 impossible

As Comparative Examples 2-1 and 2-2 to be compared with Examples 2-1 and2-2, the battery elements 20 were produced in a manner similar toExamples 2-1 and 2-2, except that the value of z in (98−z):z:2, which isa mass ratio of the fired material of LiFePO₄ containing a carbonmaterial to ammonium salt polyacrylic acid to styrene butadiene rubberlatex, was varied as shown in Table 2. The peel strength tests were alsoconducted for Comparative Examples 2-1 and 2-2 in a manner similar toExamples 2-1 and 2-2. Obtained results are shown in Table 2.

As evidenced by Table 2, sufficient binding force was obtained inExamples 2-1 and 2-2. In Comparative Example 2-1, however, the bindingforce was insufficient. It is believed, for example, that this resultedbecause the content of ammonium polyacrylic acid was too small. InComparative Example 2-2, gelation of the cathode mixture wassignificant, so that application to the cathode current collector layer21 a was impossible. It is believed, for example, that this resultedbecause the content of ammonium polyacrylic acid was too much.

In this regard, it was found that by using ammonium salt polyacrylicacid and styrene butadiene rubber latex as a binder, and setting thecontent of ammonium salt polyacrylic acid to the range from about 0.5 wt% to about 2.5 wt %, thickening effect was properly given to the cathodemixture, and sufficient binding force could be obtained.

Examples 3-1 and 3-2

The battery elements 20 were produced in a manner similar to Example1-1, except that a mass ratio of a fired material of LiFePO₄ containinga carbon material to ammonium salt polyacrylic acid to styrene butadienerubber latex, was set to 97:1:2. Subsequently, the exterior members 30 aand 30 b made of aluminum laminated films were prepared, the adhesivefilm 31 was arranged between the cathode lead wire 11 and the anode leadwire 12 and the exterior members 30 a/30 b, and the battery elements 20were vacuum packaged. In this manner, the secondary batteries wereassembled.

Charge and discharge cycle tests and high load discharge tests wereconducted for the secondary batteries of Examples 3-1 and 3-2 producedas above. Obtained results are shown in Table 3.

The charge and discharge cycle tests were conducted as follows. First,charging was performed with a constant current of 100 mA until thecharge final voltage shown in Table 3 was attained. After that,discharging was performed with a constant current of 100 mA until thebattery voltage reached 2.0 V. Charge and discharge were repeated underthe same conditions, and the discharge capacity of the 10th cycle,wherein the charge and discharge capacity settled was measuredrespectively.

TABLE 3 Discharge 1000 mA capacity high load Cathode Charge final of the10^(th) discharge active Binder voltage cycle capacity material Binder(wt %) (V) (mAh) (mAh) Example 3-1 LiFePO₄ SBR + PAA 3 3.6 519 421Example 3-2 LiFePO₄ SBR + PAA 3 4.0 523 413 Comparative LiFePO₄ SBR +PAA 3 4.2 273 138 Example 3-1 Comparative LiCoO₂ PVDF 6 4.2 485 342Example 3-2

High load charge and discharge tests were conducted as follows. First,charging was performed with a constant current of 100 mA until thecharge final voltage shown in Table 3 was attained. After that,discharging was performed with a constant current of 1000 mA until thebattery voltage reached 2.0 V, and respective discharge capacities weremeasured.

As to the secondary batteries of Comparative Example 3-1 to be comparedwith Examples 3-1 and 3-2, made in a manner similar to Example 3-1, thecharge and discharge cycle tests and the high load charge and dischargetests were conducted in a manner similar to Examples 3-1 and 3-2, exceptthat the charge final voltage was 4.2 V. Obtained results were shown inTable 3. As to the secondary batteries of Comparative Example 3-2 to becompared with Examples 3-1 and 3-2, made in a manner similar to Example3-1, except that LiCoO₂ was used as a cathode active material, andpolyvinylidene fluoride of 6 wt % was used as a binder at a mass ratioof 94:6, the charge and discharge cycle tests and the high load chargeand discharge tests were conducted in a manner similar to Examples 3-1and 3-2, except that the charge final voltage was 4.2 V. Obtainedresults are shown in Table 3. The cathode mixture of Comparative Example3-2 is a conventionally typical example.

As evidenced by Table 3, according to Examples 3-1 and 3-2, thedischarge capacities of the 10th cycle were high, such as 519 mAh andabove, and the high load discharge capacities of 1000 mA were high, suchas 413 mA and above. Meanwhile, in Comparative Example 3-1, thedischarge capacity of the 10th cycle was 273 mAh and the high loaddischarge capacity was 138 mAh, such as, both capacities were very low.In this regard, it was shown that in Examples 3-1 and 3-2, the cyclecharacteristics and the high load discharge capacity were improved. Itis believed, for example, that when charge OCV (open circuit voltage)has a potential of 4.0 V and under, ammonium salt polyacrylic acid andstyrene butadiene rubber latex were not decomposed, and when itspotential was 4.2 V and above, ammonium salt polyacrylic acid andsynthetic rubber latex were decomposed. In Comparative Example 3-2, thecontent of the binder was twice as large as in Examples of 3-1 and 3-2,and the discharge capacity of the 10th cycle was not low, such as 485mAh, but the high load discharge capacity was low, such as 342 mAh. Inthis regard, it was confirmed that in the case where ammonium saltpolyacrylic acid and styrene butadiene rubber latex were used as abinder, even when the total amount of the binder was decreased,excellent cycle characteristics were obtained, and the high loaddischarge capacity was improved.

In this regard, it was confirmed that when the charge final voltage wasnot over 4.0 V, ammonium salt polyacrylic acid and styrene butadienerubber latex could be used as a binder, so that the content of thebinder can be lowered, and the cycle characteristics and the high loaddischarge capacity were improved.

In Examples 1-1 to 3-2, ammonium salt polyacrylic acid was used as athickener. However, the same effect was confirmed when natrium saltpolyacrylic acid, ammonium salt carboxymethyl cellulose, or natrium saltcarboxymethyl cellulose was used.

In Examples 1-1 to 3-2, LiFePO₄ was used as a cathode active material.However, when using other systems whose charge final voltage was notover 4.0 V, such as LiFe_(0.2)Cu_(0.8)PO₄, LiFe_(0.9)Ti_(0.1). PO₄,LiFe_(0.8)Zn_(0.2) PO₄, LiFe_(0.8)Mg_(0.2)PO₄ and the like as a cathodeactive material, similar effect was confirmed.

The results of Examples 1-1 to 3-2 demonstrate that by using a syntheticrubber latex adhesive, such as styrene butadiene rubber latex, and athickener, such as ammonium salt polyacrylic acid, as a binder of thecathode 21 in the secondary battery with its charge final voltage ofabout 4.0 V or less, the cycle characteristics and the high loaddischarge capacity can be improved. In this regard, it was found thatthe secondary battery, wherein the charge and discharge capacity, thecapacity maintenance ratio, and the discharge cycle life were improved,particularly the load characteristics was improved, could be obtained.

In the above examples, descriptions were made on materials for thesynthetic rubber latex adhesive and the thickener with concreteexamples. However, similar results can also be obtained and contemplatedwhen using materials with other suitable structures according to anembodiment of the present invention.

Examples 4-1 and 4-2

The battery elements 20 were produced in a manner similar to Example1-1, except that a cathode mixture was prepared as follows. First, acathode mixture was prepared by sufficiently kneading a fired materialof LiFePO₄ containing a carbon material, and polyvinylidene fluoridedenaturalized by maleic acid, and partly substituted with hexafluoropropylene (HFPr) (hereinafter referred to as HFPr-substituted maleicacid-denaturalized PVDF). Here, amount denaturalized by maleic acid inHFPr-substituted maleic acid-denaturalized PVDF was 0.3 wt %, and amountof hexafluoro propylene used for hexafluoro propylene substitution(hereinafter referred to as HFPr-substituted amount) was 3 wt %. A massratio of the fired material of LiFePO₄ containing the carbon materialand the binder comprised of HFPr-substituted maleic acid-denaturalizedPVDF was set to (100−p):p. The value of p was changed as shown inExamples 4-1 and 4-2 in Table 4. The peel strength tests were conductedin a manner similar to Example 1-1, for the battery elements 20 ofExamples 4-1 and 4-2 produced as above. Obtained results are shown inTable 4. The peel strength tests in Table 4 were evaluated in a mannersimilar to Table 1.

TABLE 4 Maleic acid- HFPr- Binder denaturalized substituted mass Peelstrength Binder amount (wt %) amount (wt %) ratio p test Example 4-1HFPr-substituted maleic 0.3 3 0.5 Sufficient acid-denaturalized PVDFExample 4-2 HFPr-substituted maleic 0.3 3 4 Strong acid-denaturalizedPVDF Comparative HFPr-substituted maleic 0.3 3 0.3 Insufficient Example4-1 acid-denaturalized PVDF Comparative HFPr-substituted maleic 0.3 3 5Measurement Example 4-2 acid-denaturalized impossible PVDF ComparativePVDF — — 3 Insufficient Example 1-3 Comparative PVDF — — 6 SufficientExample 1-4

As Comparative Examples 4-1 and 4-2 to be compared with Examples 4-1 and4-2, the battery elements 20 were produced in a manner similar toExamples 4-1 and 4-2 except that the value of p in (100−p):p, a massratio of a fired material of LiFePO₄ containing a carbon material and abinder made of HFPr-substituted maleic acid-denaturalized PVDF wasvaried as shown in Table 4. The peel strength tests were also conductedfor Comparative Examples 4-1 and 4-2 in a manner similar to Examples 4-1and 4-2. Obtained results are shown in Table 4. As comparative examplesto Examples 4-1 and 4-2, the results of Comparative Examples 1-3 and 1-4are also shown in Table 4. The polyvinylidene fluoride used inComparative Examples 1-3 and 1-4 was neither denaturalized by maleicacid nor substituted with hexafluoro propylene.

As evidenced by Table 4, sufficient binding force was obtained inExample 4-1, and strong binding force was obtained in Example 4-2. InComparative Example 4-1, however, sufficient binding force could not beobtained. It is believed, for example, this was due because the amountof HFPr-substituted maleic acid-denaturalized PVDF as the binder was toosmall. In Comparative Example 4-2, gelation of the cathode mixture wassignificant, so that application to the cathode current collector layer21 a was not performed well. It is believed, for example, this was duebecause the amount of HFPr-substituted maleic acid-denaturalized PVDF asthe binder was too much. In Comparative Example 1-3 containingpolyvinylidene fluoride of 3 wt %, no binding force was shown, anddrastic peeling appeared at the electrode. In Comparative Example 4-4containing polyvinylidene fluoride of 6 wt %, though sufficient bindingforce was obtained, the content of the binder was too much.

In this regard, it was found that by using HFPr-substituted maleicacid-denaturalized PVDF as a binder and setting a content ofHFPr-substituted maleic acid-denaturalized PVDF ranges from about 0.5 wt% to about 4 wt %, a content of binder became small, and the cathode andthe secondary battery with sufficient binding force, i.e. high strengthwere obtained.

Examples 5-1 to 5-2

The rolled battery elements 20 were produced in a manner similar toExample 4-1, except that an amount denaturalized by maleic acid ofHFPr-substituted maleic acid-denaturalized PVDF was varied as shown inTable 5, and a mass ratio of a fired material of LiFePO₄ containing acarbon material to HFPr-substituted maleic acid-denaturalized PVDF wasset to 98:2. The peel strength tests were conducted for the batteryelements 20 of Examples 5-1 and 5-2 made as above in a manner similar toExample 4-1. Obtained results are shown in Table 5. The peel strengthtests in Table 5 were evaluated in a manner similar to Table 4.

TABLE 5 Maleic acid- Binder denaturalized HFPr-substituted mass Peelstrength Binder amount (wt %) amount (wt %) ratio p test Example 5-1HFPr-substituted 0.1 3 2 Sufficient maleic acid- denaturalized PVDFExample 5-2 HFPr-substituted 0.4 3 2 Strong maleic acid- denaturalizedPVDF Comparative HFPr-substituted 0.05 3 2 Insufficient Example 5-1maleic acid- denaturalized PVDF Comparative HFPr-substituted 0.5 3 2Measurement Example 5-2 maleic acid- impossible denaturalized PVDF

As Comparative Examples 5-1 and 5-2 to be compared with Examples 5-1 and5-2, the rolled battery elements 20 were produced in a manner similar toExample 4-1, except that an amount denaturalized by maleic acid ofHFPr-substituted maleic acid-denaturalized PVDF was varied as shown inTable 5, and a mass ratio of a fired material of LiFePO₄ containing acarbon material to HFPr-substituted maleic acid-denaturalized PVDF wasset to 98:2. The peel strength tests were also conducted for ComparativeExamples 5-1 and 5-2 in a manner similar to Examples 5-1 and 5-2.Obtained results are shown in Table 5.

As evidenced by Table 5, sufficient binding force was obtained inExamples 5-1, and strong binding force was obtained in Example 5-2. InComparative Example 5-1, however, the binding force was insufficient andthe cathode 21 with insufficient strength was obtained. It is believed,for example, this resulted because the amount denaturalized by maleicacid was too small. In Comparative Example 5-2, gelation of the cathodemixture was significant, so that application to the cathode currentcollector layer 21 a was impossible. It is believed, for example, thisresulted because the amount denaturalized by maleic acid was too much.

In this regard, it was found that by using HFPr-substituted maleicacid-denaturalized PVDF as a binder and the maleic acid-denaturalizedamount of HFPr substituted-maleic acid-denaturalized PVDF that rangesfrom about 0.1 wt % to about 0.4 wt %, the cathode with sufficientbinding force, such as high strength and the secondary battery wereobtained, and reduction of the amount of binder was supported.

Examples 6-1 to 6-3

Cathode mixture was prepared by setting a mass ratio of a fired materialof LiFePO₄ containing a carbon material, to maleic acid-denaturalizedpolyvinylidene fluoride (hereinafter referred to as maleicacid-denaturalized PVDF) to 98:2. The maleic acid-denaturalized amountof maleic acid-denaturalized PVDF was 0.3 wt %, and amount of hexafluoropropylene used for hexafluoro propylene substitution (hereinafterreferred to as HFPr-substituted amount) was varied as shown in Table 6.Then, the battery elements 20 were formed in a manner similar to Example4-1 except for the above.

TABLE 6 1000 mA Discharge high HFPr- capacity load substituted Peel ofthe 10th discharge amount Binder strength cycle capacity (wt %) (wt %)test (mAh) (mAh) Example 6-1 5 2 Strong 521 418 Example 6-2 Nosubstitution 2 Strong 509 398 Example 6-3 6 2 Strong 295 152

The peel strength tests were conducted in a manner similar to Example4-1 for the battery elements 20 of Examples 6-1 to 6-3 made as above.Obtained results are shown in Table 6. The peel strength tests in Table6 were evaluated in a manner similar to Table 4.

Further, the external members 30 a and 30 b made of aluminum laminatedfilms were prepared, adhesive film 31 was arranged between the cathodelead wire 11 and anode lead wire 12 and the external materials 30 a and30 b, and the battery elements 20 of Examples 6-1 to 6-3 produced asabove were vacuum packaged. In this way, the secondary battery wasassembled.

The charge and discharge cycle tests and the high load discharge testswere conducted for the secondary batteries of Examples 6-1 to 6-3 madeas above, in a manner similar to Examples 3-1 and 3-2. Obtained resultsare shown in Table 6.

As evidenced by Table 6, strong binding force was obtained in any ofExamples 6-1 to 6-3. In this regard, it was found that by using maleicacid-denaturalized PVDF or HFPr-substituted maleic acid-denaturalizedPVDF as a binder, a content of the binder became less, and the cathodeand the secondary battery with sufficient binding force, such as highstrength, were obtained.

Further, as evidenced by Table 6, according to Examples 6-1 and 6-2, thedischarge capacities of the 10th cycle were high, such as 509 mAh andabove, and the high load discharge capacities of 1000 mA were high, suchas 398 mAh and above. Meanwhile, in Example 6-3, the discharge capacityof the 10th cycle was 295 mAh and the high load discharge capacity was152 mAh, such as both capacities were low.

In this regard, it was confirmed that by using HFPr-substituted maleicacid-denaturalized PVDF whose HFPr-substituted amount of 5 wt % andunder, the cycle characteristics and the high load discharge capacitywere improved.

Examples 7-1 and 7-2

The secondary batteries were produced in a manner similar to Example6-1, except that HFPr-substituted maleic acid-denaturalizedpolyvinylidene fluoride of 2 wt % whose maleic acid-denaturalized amountwas 0.3 wt % and HFPr-substituted amount was 3 wt %, was used as abinder.

The charge and discharge cycle tests and the high load discharge testswere conducted for the secondary batteries of Examples 7-1 and 7-2produced as above. Obtained results are shown in Table 7. The charge anddischarge cycle tests and the high load discharge tests were conductedin a manner similar to Examples 6-1 and 6-2, except that the chargefinal voltage was varied as shown in Table 7.

TABLE 7 Discharge Charge capacity of 1000 mA high Cathode final the 10thload discharge active Binder voltage cycle capacity material Binder (wt%) (V) (mAh) (mAh) Example 7-1 LiFePO₄ HFPr-substituted 2 3.6 522 420maleic acid- denaturalized PVDF Example 7-2 LiFePO₄ HFPr-substituted 24.0 528 417 maleic acid- denaturalized PVDF Comparative LiFePO₄HFPr-substituted 2 4.2 276 140 Example 7-1 maleic acid- denaturalizedPVDF Comparative LiCoO₂ PVDF 6 4.2 502 342 Example 7-2

As to the secondary battery of Comparative Example 7-1 to be comparedwith Examples 7-1 and 7-2, made in a manner similar to Example 7-1, thecharge and discharge cycle tests and the high load discharge tests wereconducted in a manner similar to Examples 7-1 and 7-2, except that thecharge final voltage was 4.2 V. Obtained results are shown in Table 7.As Comparative Example 7-2 to be compared with Examples 7-1 and 7-2, asecondary battery was produced in a manner similar to ComparativeExample 3-2. Then, the charge and discharge cycle tests and the highload discharge tests were conducted for the secondary battery of thisComparative Example 7-2, in a manner similar to Examples 7-1 and 7-2,except that the charge final voltage was set to 4.2 V. Obtained resultsare shown in Table 7.

As evidenced by Table 7, according to Examples 7-1 and 7-2, thedischarge capacities of the 10th cycle were high, such as 522 mAh andabove, and the high load discharge capacities of 1000 mA were high, suchas 417 mAh and above. Meanwhile, in Comparative Example 7-1, thedischarge capacity of the 10th cycle was 276 mAh and the high loaddischarge capacity was 140 mAh, such as both capacities were very low.In this regard, it was shown that in Examples 7-1 and 7-2, the cyclecharacteristics and the high load discharge capacity were improved. Itis believed, for example, that when charge OCV has a potential of 4.0 Vand under, maleic acid-denaturalized PVDF was not decomposed, and whenits potential was 4.2 V and above, maleic acid-denaturalized PVDF wasdecomposed. In Comparative Example 7-2, the amount of the binder wasthree times as large as in Examples 7-1 and 7-2, and the dischargecapacity of the 10th cycle was not low, such as 502 mAh, but the highload discharge capacity was low, such as 342 mAh. In this regard, it wasconfirmed that in the case where maleic acid-denaturalized PVDF was usedas a binder, even when the total amount of the binder was decreased,excellent cycle characteristics were obtained, and the high loaddischarge capacity was improved.

Further, it was confirmed that when the charge final voltage was notover 4.0 V, maleic acid-denaturalized PVDF could be used as a binder, sothat a content of the binder can be lowered, and the cyclecharacteristics and the high load discharge capacity were improved.

In Examples 4-1 to 7-2, LiFePO₄ was used as a cathode active material.The same effect was confirmed when other system with final chargevoltage of 4.0 V and under, such as LiFe_(0.2)Cu_(0.8)PO₄,LiFe_(0.9)Ti_(0.1)PO₄, LiFe_(0.8)Zn_(0.2)PO₄, LiFe_(0.8)Mg_(0.2)PO₄, andthe like was used as a cathode active material.

Therefore, it was found that, according to Examples 4-1 to 7-2, by usingmaleic acid-denaturalized PVDF as a binder of the cathode 21 in thesecondary battery with its charge final voltage of 4.0 V and under, thecathode 21 with high strength could be obtained. Further, it was foundthat the cycle characteristics and the high load discharge capacity wereimproved. In particular, it was found that by using HFPr-substitutedmaleic acid-denaturalized PVDF as a binder of the cathode 21, the cyclecharacteristics and the high load discharge capacity were furtherimproved. In this regard, it was found that the charge and dischargecapacity, the capacity maintenance ratio, and the discharge cycle lifewere improved, and in particular, the secondary battery with improvedload characteristics could be obtained.

As described above, the present invention, in an embodiment, includeslithium iron phosphorus oxide as a cathode active material. However,similar results can be obtained when any suitable cathode activematerial which can realize the secondary battery with its charge finalvoltage of about 4.0 V or less, is used. Further, as described above,the present invention, in an embodiment, includes the secondarybatteries using lithium as electrode reactive species. However, thesimilar result can be obtained when any suitable cathode activematerial, which can realize the secondary battery with its charge finalvoltage of 4.0 V and under, is used.

Though the invention has been described by the embodiment and theexamples, the present invention is not limited to the embodiment andexamples but can be modified in any various and suitable manner. Forexample, the battery element is enclosed inside of the exterior membersmade of aluminum laminated films in the embodiment and examples but, itcan be enclosed inside of the exterior members made of other laminatedfilms in an embodiment.

Further, the gel electrolyte is used in an embodiment and examplesdiscussed above, but other electrolytes, for example, electrolytesolution, i.e. the liquid electrolyte, a solid high molecular weightelectrolyte which is obtained by dispersing electrolyte salt in a highmolecular weight compound with ion conductivity, a solid inorganicelectrolyte or the like can be used in an embodiment.

For the solid electrolytes as the high molecular weight compounds, forexample, either high molecular weight compounds such as polyethyleneoxide or cross-linked polymer including polyethylene oxide, ester highmolecular weight compounds such as polymethacrylate, or acrylate highmolecular weight materials or the like can be used independently, bymixture, or in copolymerization in molecules in an embodiment. Further,as an inorganic conductor, polycrystal of lithium nitride, lithiumiodide, or lithium hydroxide; a mixture of lithium iodide and dichromiumtrioxide, or a mixture of lithium iodide, lithium sulfide; anddiphosphorus subsulfide; and the like in an embodiment can be used.

Further, the cathode and the anode are wound in the embodiment andexamples described above but, the cathode and the anode can be folded,piled or configured in other suitable manners.

Further, an example about the secondary battery whose battery elementwith winding structure is enclosed inside the exterior members isspecifically described in an embodiment and examples as discussed above.However, the invention can be applied to secondary batteries havingother structures. Additionally, the present invention can be similarlyapplied to a cylindrical secondary battery, secondary batteries havingother shapes, such as a coin shape, a button shape, a square shape orthe like.

As described above, according to the cathode or the battery pursuant toan embodiment of the present invention, effective amounts of thesynthetic rubber latex adhesive and the thickener are contained in thecathode mixture layer as a binder, so that flexibility and smoothnesscan be improved, and electrode peeling and cracking can be prevented.Further, effective amounts of the synthetic rubber latex adhesive andthe thickener, or maleic acid-denaturalized polyvinylidene fluoride iscontained, so that a content of the binder is lowered to increase ratioand capacity of the cathode active material, and electron transfer inthe cathode can be facilitated. Thus, the charge and discharge capacityand the charge and discharge cycle life can be improved, and the loadcharacteristics can be improved.

According to the cathode or the battery pursuant to an embodiment of thepresent invention, a lithium phosphorous oxide having an olivinestructure is contained as a cathode active material, so that excellentbattery characteristics can be obtained when the charge final voltage is4.0 V and under.

Further, according to the cathode or the battery pursuant to anembodiment of the present invention, an effective amount of a carbonmaterial is contained in the cathode active material, so that excellentconductivity can be obtained, and the load characteristics and thecharge and discharge capacity can be further improved.

Furthermore, according to the cathode or the battery pursuant to anembodiment of the present invention, an amount of maleicacid-denaturalized polyvinylidene fluoride ranges from about 0.1 wt % toabout 0.4 wt %, so that electrode peeling can be prevented moreeffectively.

Additionally, according to the cathode or the battery pursuant to anembodiment of the present invention, a part of maleic acid-denaturalizedpolyvinylidene fluoride is substituted with hexafluoro propylene and hasa substitution ratio of about 5 wt % or less. Therefore, the cyclecharacteristics and the high load discharge capacity can be furtherimproved.

It should be understood that various charges and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present invention andwithout diminishing its intended advantages. It is therefore intendedthat such changes and modifications be covered by the appended claims.

1. A cathode comprising: a cathode mixture layer including a cathodeactive material and a binder, the binder including a styrene butadienelatex adhesive and a thickener wherein the content of the styrenebutadiene latex adhesive ranges from about 2 wt % to about 4 wt % of thetotal mass of the mixture cathode layer, the content of the thickenerranges from about 0.5 wt % to about 2.5 wt % of the total mass of thecathode mixture layer, and the thickener is polyacrylic acid, andwherein the cathode active material consists of a lithium ironphosphorous oxide and a carbon material, the lithium iron phosphateoxide having an olivine structure and the content of the carbon materialbeing in the range of from about 5 wt % to about 12 wt % with respect tothe total amount of the cathode active material.
 2. The cathode of claim1, wherein the ratio of styrene-butadiene latex adhesive to polyacrylicacid is between about 0.8:1 to about 4:1 by mass.
 3. The cathode ofclaim 1 wherein the styrene-butadiene latex adhesive and polyacrylicacid represent greater than 2.3% weight and less than 6% weight of thecathode mixture layer.
 4. The cathode of claim 1 wherein thestyrene-butadiene latex adhesive and polyacrylic acid represent fromabout 2.5% to about 5% weight of the cathode mixture layer.
 5. A batterycomprising: a cathode, the cathode including a cathode mixture layercontaining a cathode active material, and a binder including a styrenebutadiene latex adhesive and a thickener; an anode; and an electrolyte,wherein the content of the styrene butadiene latex adhesive ranges fromabout 2 wt % to about 4 wt % of the total mass of the cathode mixturelayer, wherein the content of the thickener ranges from about 0.5 wt %to about 2.5 wt % of the total mass of the cathode mixture layer and thethickener is polyacrylic acid, and wherein the battery has a chargefinal voltage of about 4.0 V or less, and wherein the cathode activematerial consists of a lithium iron phosphorous oxide and a carbonmaterial, the lithium iron phosphate oxide having an olivine structureand the content of the carbon material being in the range of from about5 wt % to about 12 wt % with respect to the total amount of the cathodeactive material.
 6. The battery of claim 5, wherein the ratio ofstyrene-butadiene latex adhesive to polyacrylic acid is between about0.8:1 to about 4:1 by mass.
 7. The battery of claim 5 wherein thestyrene-butadiene latex adhesive and polyacrylic acid represent greaterthan 2.3% weight and less than 6% weight of the cathode mixture layer.8. The battery of claim 5 wherein the styrene-butadiene latex adhesiveand polyacrylic acid represent from about 2.5% to about 5% weight of thecathode mixture layer.